I was working for Amtrak during that era and the MBTA had the final say on what operated where. Some of the high maintenance equipment was moved to the North Station routes as there was an enclosed site where they could be maintained. Also, the passenger volumes out of South Station were astronomically higher than out of North Station, so longer trains, or higher capacity - bilevel equipment- were necessary. Some North Station lines could not use bilevels until some clearances had been improved.
Note that during the summer of 1977, just after the MBTA bought all the B&M's commuter routes and equipment, they removed all the underbody housings protecting the diesel prime movers. This is why the winter of 1978 reduced almost all of them to a single functioning prime mover which idled to produce electricity and heat for the car.
The MBTA acquired several groups of serviceable RDCs after the B&M puchase - exx-NH and exx-NYC units from Conrail in 1978, several ex-CPR between 1976 (per Duke & Keilty) and 1985, several exx-RDG, ex-SEPTA in 1985 and one from B&O in 1984. They also leased 9 from NJT (mostly exx-P-RSL) in 1988.
Doug Kydd discusses late uses of RDC in his autobiography, but I'll have to figure out where I filed it to tell you the title/publisher.
